1. Field of the Invention
The present invention generally relates to a method of decomposing nitrogen oxides in an exhaust gas into harmless materials.
2. Description of the Prior Art
With industrial developments, pollution problems have received great attention from the viewpoint of environmental conservation. With regard to air pollution, materials other than the normal components of air, such as sulfur oxides and smoke dust, as well as nitrogen oxides, hydrocarbons, and fluorocarbons (which were considered to be inactive materials) are now considered to be harmful materials which cause air pollution. Thus, in industry, it is required to exhaust industrial waste gases after purifying the waste gases or after converting harmful materials contained in the waste gases into harmless materials (e.g., nitrogen, oxygen, water, and carbon dioxide).
Under these circumstances, the decontamination of exhaust gases containing nitrogen oxides has encountered various problems in establishing technology therefor which is economical.
In general, as nitrogen oxides, there are various kinds of nitrogen compounds, such as nitrogen dioxide which shows strong toxicity, and stable nitrogen monoxide, but since at high temperature combustion nitrogen monoxide exists in a stable state, nitrogen oxides exist almost entirely as the state of nitrogen monoxide in combustion gases.
As purification methods for exhaust gases containing nitrogen monoxide, a wet method, a dry method, etc., have been proposed, but since nitrogen monoxide is a stable product, an effective purification technique therefor has not yet been established.
That is, in conventional purification methods, there are various problems,
E.G., IN THE WET METHOD, TREATMENT OF THE WASTE SOLUTION USED IN THE WET METHOD IS ADDITIONALLY REQUIRED, AND IN THE DRY METHOD, WHICH EMPLOYS A CATALYST, THE LIFE OF THE CATALYST IS GREATLY INFLUENCED BY SULFUR COMPOUNDS AND DUST CONTAINED IN THE EXHAUST GAS. Among these purification methods, non-catalytic dry methods of removing nitrogen oxides have been proposed by Myerson in Japanese Patent Application (Laid Open) 47,244/74 and Lyon in Japanese Patent Publication 35,908/75 and are noticeable techniques from the viewpoint of the cost of equipment and the economy of operation.
A combustion exhaust gas contains generally, as a component thereof, about 0.3 to about 10% by volume of oxygen since a larger amount of air than that required for burning fuels is supplied to a combustion zone. On the other hand, the content of nitrogen oxides in the combustion exhaust gas is about 50 to about 10,000 ppm by volume (hereinafter only referred to as "ppm"), even more generally 80 to 2,000 ppm.
When such a combustion exhaust gas containing a large amount of oxygen is treated by the method proposed by Myerson, a large amount of a reducing agent is inevitably consumed by reaction with oxygen since the reaction rate of the reducing agent and oxygen is higher than the reaction rate of the reducing agent and nitrogen oxide. On the other hand, when the combustion exhaust gas is treated by the method proposed by Lyon, the consumption of the reducing agent may be small since the reaction of the reducing agent and nitrogen oxides selectively proceeds faster than the reaction of the reducing agent and oxygen, but the treatment is accompanied by the faults that the reaction temperature range employed is narrow and, in order to enlarge the reaction temperature range, a large amount of reducing agent including hydrogen, hydrocarbons, etc., must be used.
Furthermore, if in the case of using ammonia as the reducing agent the ammonia is exhausted from the system in the unreacted state, there is the possibility of secondary pollution, and, further, in a system where the exhaust gas contains sulfur oxides, the combustion system may encounter clogging problems due to the deposition of ammonium hydrogen sulfate. In general, the oxidative decomposition of ammonia is carried out at high temperatures in the presence of catalyst. Thus, it is difficult to decompose unreacted ammonia in a non-catalytic system or the use of catalyst is inevitable in such a system, which causes the aforementioned troubles.